The invention pertains generally to a stress limiter apparatus for a gas turbine engine and is more particularly directed to a stress limiter apparatus for an acceleration based closed loop fuel control system of a gas turbine engine.
The closed loop fuel control of gas turbine engines based on acceleration has been successfully implemented in various systems. These systems act directly to control the rate of change of the engine speed as a function of an acceleration term. The acceleration term is formed by differencing a scheduled term with an actual or an implied actual parameter of the gas generator. The actual acceleration of the engine is fed back through changes in the actual term for comparison with the scheduled term. Thus, the acceleration term can be envisioned as the difference between a desired or scheduled acceleration and the actual acceleration. The scheduled acceleration may be a function of any of a number of engine operating parameters including engine speed, ambient pressure, temperature, compressor pressure, etc.
After the acceleration term is formed, it is integrated and the result used to effectively provide a datum for a proportional speed control loop. The speed control loop meters fuel to the engine based upon a proportional multiplication of the difference between the actual engine speed and the integrated acceleration term. Advantages of this form of acceleration control include consistent predictible accelerations which take account of fuel type, temperature, and altitude. The acceleration time of these controls is also generally independent of air bleed and power extraction status.
A closed loop fuel control for a gas turbine engine based on acceleration is more fully disclosed in a U.S. patent application Ser. No. 210,938, filed in the name of Rowland M. Evans, on Nov. 28, 1980, which is commonly assigned with the present application. The disclosure of Evans is hereby expressly incorporated by reference herein. Other examples of closed loop systems of this type are illustrated in U.S. Pat. Nos. 4,018,044; 4,100,731; and 4,040,250.
The acceleration that is requested should be a function of the final desired output power or speed. Generally, it is desired that an acceleration be the safe maximum available from the engine for the particular operating conditions of the engine. Only the physical limitations of the engine should reduce the acceleration capability of the control.
One set of limiting conditions is where the compressor begins to stall and the engine enters a surge condition destructive to the equipment. Therefore, the acceleration and fuel flow must be controlled so that a safe operating margin is maintained between stall conditions and the operating point so the engine does not encounter these problems. On most controls an acceleration limiter, providing predetermined acceleration limits for the acceleration term, is utilized to control the safety factor. When on acceleration limit, the fuel flow to the engine is scheduled by the acceleration limiter without regard to the steady running line of the engine.
However, there are other parameters besides the acceleration limit for the demanded acceleration term that must be controlled to safely operate a gas turbine engine. These include parameters that are related to the physical stress limits of the particular engine. For example, in turbojet engines the turbine temperature should not exceed a designed maximum or engine failure could occur. This is because the turbine blades have a critical temperature which can be exceeded by the maximum gas temperature. Another critical factor is overspeed of the engine beyond a designed maximum. The excess centrifugal forces developed during overspeed can cause the blades on a compressor or turbine to deform or break off. Therefore, if either of these limits is exceeded by excessive margins for even relatively short periods of time, engine damage or complete failure can result.
Other stress limits that must be controlled closely if the gas turbine engine is of the free turbine type, are the free turbine overspeed limit and the free turbine output shaft and/or transmission gearbox torque limit. As with the other engine stress limits, these limits are imposed because the engine can produce forces which exceed the mechanical strength of the free turbine and its rotor shaft, or which can reduce its life.
To prevent overstress of the engine, conventional closed loop fuel controls for gas turbine engines compare a particular stress parameter against a predetermined constant limit value and generate a signal indicating when the limit is exceeded. When the engine is overstressed as indicated by the signal, the fuel flow is reduced by manipulation of the scheduled operating point for the gas generator governor or the acceleration schedule. Both of these techniques entail a delay between the time the limiting parameter is exceeded and the fuel flow is reduced below the engine steady running line. The delay may cause a transient overshoot of the stress parameter beyond its limit and unnecessary overstressing of engine components.
However, if the comparison limit is set below the actual physical value of the stress limit to prevent overshoot, the engine is better protected but at the cost of system response. Therefore, it would be highly desirable to provide a stress limiter apparatus for a gas turbine engine control that not only would eliminate transient overshoot during stress limiting, but also would allow the control to operate as close to the actual stress limit as possible for maximum performance.